1. Field of the Invention
The present invention relates to an electric power steering unit of a vehicle provided with an electric motor (simply referred to as motor as well) that provides an assist power to a turning operation of a steering handle and, more particularly, to an electric power steering unit that prevents a motor or an ECU (ELECTRONIC CONTROL UNIT) carrying out a driving control of this motor from being overheated by limiting a current flowing through the motor.
2. Description of the Related Art
As a conventional motor control unit that controls a motor while monitoring the temperature rise of a motor (i.e., electric motor) for use in an electric power steering unit, for example, the Japanese Patent No. 2892899 discloses a motor control unit.
Specifically, in the Japanese Patent No. 2892899, the following “motor control unit” is disclosed. “This motor control unit applies an electric power to a motor in response to the power application operation from outside, and is provided with a control section for setting various control information with respect to the motor. The motor control unit includes: power supply self-hold means for self-holding a power supply to be supplied to this control section; temperature estimating means for estimating an elevated temperature of the motor with a value of current flowing through the motor; interruption time period operation means for operating a time period before interruption of power supply having been self-held based on the elevated temperature having been estimated after the interruption of power supply to the motor; and power supply interruption means for outputting a power supply interruption command to the power supply self-hold means after an interruption time period having been operated has passed.”
In this Japanese Patent No. 2892899, it is further described that a power supply of the motor control unit is held and control information having been set in the unit is maintained until an elevated temperature of the motor falls to not more than a permitted value after the interruption of power supply of the motor (electric motor), thereby enabling to achieve the protection from overheat and improvements in restart properties.
In another Japanese Patent No. 3405292, the following “Electric Power Steering Unit of Vehicle” is disclosed. “This electric power steering unit of vehicle includes: an electric motor for providing an assist power to the turning operation of a steering handle; assist current value determination means for determining an assist current value in accordance with a steering state of the steering handle; final assist current value determination means that updates a current limit value so as to make this current limit value smaller when a value indicating a heated state of an electric motor to be updated in accordance with a current-conducting state of the electric motor is larger than a motor current limit determined value, and that limits an assist current value with this current limit value to determine a final assist current value; storage means for storing data without being supplied with an electric power; initial value setting means that causes storage means to store a ratio between a value indicating an overheat state of the electric motor and the maximum value of the above-mentioned motor current limit determined value as data regarding a current limit value when an ignition switch is changed from ON state to OFF state, and that establishes a value corresponding to the data regarding a current limit value having been stored when the ignition switch is changed from OFF state to ON state as an initial value of a value indicating an overheat state of the electric motor; and motor electrification means for conducting current through the electric motor in accordance with a final assist current value.”
In a further Japanese Patent No. 3405292, the following techniques are disclosed. That is, when the ignition switch is changed from ON state to OFF state, data regarding a current limit value can be stored without a battery being loaded. Further, the data having been stored is read out and processed when the ignition switch comes to be in ON state next, and this data is established as an initial value of a value indicating an overheat state of the electric motor to be updated in accordance with a current-conducting state of the motor. Therefore, even in the case where once the ignition switch has been changed from ON state to OFF state and thereafter is changed to ON state in a short time period, a current limit value becomes an appropriate value, thereby enabling to prevent a drive circuit or an electric motor from being over-heated.
However, in the motor control unit disclosed in the Japanese Patent No. 2892899 or the electric power steering unit of vehicle disclosed in the Japanese Patent No. 3405292, even after the ignition switch has been changed to be in OFF state, the supply of an electric power from the battery of a vehicle to the electric control unit continues until the elevated temperature of the motor (electric motor) falls to not more than a permitted value.
Consequently, a problem exists in that a load on the battery at the time of the ignition switch being changed from in state to OFF state is large.
Moreover, when the ignition switch is changed from ON state to OFF state with the motor (electric motor) still being at a high temperature, a value indicating an overheat state of the motor is stored in the storage means as data regarding the current limit value.
Accordingly, even when the ignition switch is changed from OFF state to ON state after the temperature of the motor has dropped fully to not more than a permitted value, a value based on the data regarding the current limit value having been stored at the time of the motor being at a high temperature is established as an initial value of a current limit value, and the current corresponding to this initial value flows through the motor.
Thus, even in the case where the OFF state of the ignition switch continues for a long time period, a temperature of the motor has dropped fully and an initial value of the current limit value at the time of bringing the ignition switch in ON can be made larger (that is, in the case where a motor current at the time of bring the ignition switch in ON can be larger), the initial value of the current limit value still remains at a low value having been stored.
In consequence, a further problem exists in that starting characteristic of a motor (electric motor) is restricted, and the motor is not driven efficiently with an appropriate initial value of the current limit value.